The invention concerns an improvement in enclosures in which a bed of solid particle flows, referred to as a moving bed. The invention is more particularly applied to moving-bed reactors and in particular moving-bed catalytic reactors such as those used for catalytic reforming, in which solid granular particles (of catalyst) of the same nature (a single catalyst) circulate.
In moving-bed units in which circulation of the solid, which may or may not be catalytic and which may or may not be of spherical shape is effected in the form of a dense bed which flows vertically under the effect of the force of gravity, the walls which define the granular zone most frequently comprise two cylindrical grills or gratings of different diameters. The fluid, in a liquid phase or most frequently a gaseous phase, or possibly the fluids (gas and liquid or liquid and other immiscible liquid) passes through he granular bed (which in this case is also annular) in crossed flow relationship, that is to say with a radial flow configuration, from the outside inwardly, or conversely from the inside outwardly. After having passed through the cylindrical inlet frill the fluid therefore passes through the bed and then issues from the annular space containing the granular medium by passing through a second grill, the outlet grill, which is concentric with the inlet grill.
The flow of the fluid or fluids through the bed causes a pressure drop which depends on a plurality of factors, the main ones of which are the size and the shape of the particles, the properties of the fluid or fluids and the flow rate of the fluid or fluids. That pressure drop manifests itself as a thrust force applied by the fluid against the solid particles in the direction of flow of the fluid. That thrust force which is applied towards the fluid outlet grill modifies the whole of the balance of the forces to which the solid particles (for example balls) are subjected. That phenomenon is of a nature such as to give rise to disadvantages such as a reduction in the speed of the movement of the particles which are against the wall or even jamming thereof against the outlet grill and complete stoppage thereof, which on the one hand seriously affects good systematic utilisation of those particles but which on the other hand can also cause a reduction in the speed of movement of or even stoppage of other particles, more specifically progressively involving complete blocking of the bed (for example a catalytic bed) and complete stoppage of the circulation of the solid. The severity of that phenomenon increases in proportion to an increasing thrust force on the part of the fluid or fluids and thus an increasing capacity of the unit.
That blocking phenomenon therefore constitutes a major limitation in terms of increasing the capacity of the units. It is therefore a particularly attractive proposition to use arrangements and devices which reduce the magnitude of that phenomenon.
That problem arises in acute form in catalytic units such as catalytic reforming units. The risk of the catalytic particles being jammed against the grills then results in a serious reduction in the flow rates of gas to be treated and therefore the capacities of the units.
In addition, in the course of the different treatments involved such as in particular regeneration of the catalytic particles or transfers between the reactors or between the reactors and the regenerators, fragments of balls are formed, which very quickly become jammed against the grills. That fragmented population accelerates the blocking phenomenon.